The influence of dispersal, landscape structure and catastrophic disturbance on the genetic diversity and structure of artesian spring snail populations

The outflow at some springs can be like a small creek.Some springs are characterised by having damp soil.Other springs can have deep spring pools.Location of the Hermit Hill spring complex within the Great Artesian Basin in Australia. The complex is comprised of seven previously identified spring groups: Bopeechee, Dead Boy, Hermit Hills, Old Finniss, Old Woman, Sulphuric and West Finniss.Aerial photo of the West Finniss spring group showing the patchy and clumped nature of artesian springs.The snails, Fonscochlea accepta, in a spring outflow tail.

Australia's Great Artesian Basin (GAB) is the largest artesian system in the world, covering 22% of the continent and is of national cultural, economic and biological significance. The basin supports more than 600 artesian spring areas, ranging from damp mounds to large pools with flowing drainages.

Artesian springs form when geological structures allow water from the GAB to reach the surface, forming freshwater spring pools in an otherwise inhospitable desert environment. This unique ecosystem is naturally fragmented with the springs likened to "aquatic islands in an arid sea" and they support a wealth of endemic species including plants, arachnids, crustaceans, molluscs and fish.

Our study site is the Hermit Hills spring complex in northern South Australia, located on the southwest margin of the GAB and we have focused our research on one of the endemic aquatic snails, Fonscochlea accepta. Long term monitoring of the springs means that we have data sets that are both extensive in terms of geographical sampling and for some springs, span over 20 years. We have analysed these large spatial and temporal data sets using an array of 9 microsatellite markers (ref 1).

Spatial analyses

The aquatic invertebrates that inhabit the springs are for the most part restricted to the springs and cannot survive in the surrounding desert. This limited dispersal ability suggests that population boundaries could be strongly influenced by the springs' very patchy distribution. However, our results showed that while snail populations do indeed show strong spatial structuring, dispersal occurs at two geographic scales via two very different mechanisms (refs 2,3). Short range dispersal (usually <300m) occurs via active movement along aquatic connections among springs while long-range dispersal (>3km) is likely facilitated by an animal vector (phoresy). These results underline the importance of both dispersal and landscape structure in influencing connectivity rates and patterns among populations.

Temporal analyses

Arid central Australia is subject to recurrent but irregular significant floods due to unpredictable heavy rainfall events. In 1992 our study springs were inundated by large scale flooding which decimated the snail populations. Our unique temporal data set allowed us to directly test the genetic consequences of this flood. Remarkably, our results showed that a catastrophic disturbance, which resulted in a severe population crash, lead to enhanced levels of within population genetic diversity within recovered populations. This is a unprecedented result and has important implications as to the responses of individual species to intense natural disturbances (ref 4).

Funding

Funded by: This research was supported by a collaborative Australian Research Council Linkage Grant (LP0560890) to the University of Queensland, the Queensland Museum and BHP Billiton (formerly Western Mining Corporation).